Very basic for now (only support summary card type), but I will add the 2 others type asap
I'm waiting to participate to Twitter Card to test this module, so use at your own risk ;-)

Twitter Cards support the OpenGraph meta tags, so I leverage on this metatags (og:xyz..) for the description, title, image etc ... Not sure yet if this is the good strategy or if it's better to make metatag_twittercard independant (without dependency to the metatag_opengraph module)

Give me some feedback on this module and which feature you see important,

I also started a sandbox project (see #10) to extend metatag with Twitter Cards metatag. Here is my work patched on nico059 sandbox.
This patch adds more twitter cards metatags and is using a proper metatag_twittercard_metatag_config_default_alter() function. I also added the dependency in the info file.

I'am also thinking in adding more functionality for "media" files such as images and audio/video. So that the "player" and "image" Twitter Card can be used.

The patch in #14 works partially as the "twitter:card" metatag's options and human-readable name are switched so it outputs the administrative title instead of the actual value in the metatag. Also, the site's twitter account should have its context set to to global.